Technical Field of the Invention
This invention relates generally to computing systems and more particularly to memory within such computing systems.
Description of Related Art
Computing systems are known to communicate, process, and store data. Such computing systems range from wireless smart phones, to an individual computer, to a network of computers, to data centers that support millions of web searches, stock trades, and/or on-line purchases. The data storage part of the computing system typically includes one or more memory devices corresponding to the various operational aspects of the processing and communication functions. For example, read only memory (RAM) memory is typically used for the main memory of the computing system because it can accessed in any random order with a constant response time. As another example, memory device technologies that require physical movement (e.g., magnetic disks, tapes, RAID (redundant array of inexpensive disks), and/or optical disks) are used for secondary storage of the computing system since they have a variable responses time as the physical movement can take longer than the data transfer.
Each memory device used in a computing device operates in accordance with one or more storage standards (e.g., network file system (NFS), flash file system (FFS), disk file system (DFS), small computer system interface (SCSI), internet small computer system interface (iSCSI), file transfer protocol (FTP), and web-based distributed authoring and versioning (WebDAV)). Despite standardized operations, memory devices fail; especially disk drives that are used in a stand-alone mode. For example, it is common for a commercial grade disk drive to suffer from bit level corruption during its life, which, on average, is about three years. One obvious solution for reducing disk drive failures is to use higher quality disk drives, which have a longer life, but are significantly more expensive.
Another solution for reducing the risk for losing data due a disk drive failure is to use redundant disk drives (e.g., RAID), which replicates the data into two or more copies. RAID includes an array of disk drives that store parity data and the original data. The parity data is calculated such that the failure of one or more disks will not result in the loss of the original data. For example, RAID 5 uses three or more disks to protect data from the failure of any one disc. In a three disk RAID 5 array, the parity data adds about 50% overhead to the storage of the original data. As another example, RAID 6 can recover from a loss of two disks and requires a minimum of four disks with an efficiency of (n−2)/n where n is the number of disks.
While RAID offers many advantages over stand-alone disk drivers, disk drives within RAID fail and data can be lost. For instance, as more disks are added, the probability of one or more disks failing rises, especially if lower quality disks are used. When one disk fails, if it not replaced before another disk fails, then data is lost. To reduce the risk of losing data, mirroring of RAID arrays at different physical locations is used. While this reduces the risk of losing data, it increases the vulnerability of the data to unauthorized access.